This invention relates generally to chlorine treatment of titaniferous ores containing titanium oxide and iron oxides and more particularly to a process for such treatment wherein the iron oxides within a titaniferous ore are removed by selective chloridization, and, with essentially a single chloridizing treatment, artificial rutile is economically produced as the ultimate objective product.
More specifically, the invention relates to a process for chlorine treatment of titaniferous ores which comprises a combination of a chloridization step wherein iron oxides in the titaniferous ore are selectively chloridized and removed through the use of a fluidized bed type chloridization furnace with a pretreatment step wherein the ore is heated and thereby activated and an after-treatment step wherein the chlorides, solid carbon, and gangue minerals in the ore taken out of the chloridization furnace are separated, and the TiO.sub.2 grade is elevated with the aim of ultimately producing artificial rutile of a TiO.sub.2 grade of at least 95 percent with essentially one cycle of treatment.
The term "titaniferous ore" as herein used is used to designate an ore which contains titanium dioxide, ferrous and ferric oxides, and a small quantity of coexisting substances, and which can be typically represented by ilmenite.
The method of enriching the titanium dioxide of a titaniferous ore by selectively chloridizing and thereby removing the iron oxides within the ore in a reducing atmosphere is known. For example, the specification of Australian Pat. No. 206,305 discloses a process which comprises continuously supplying a titanium-containing iron-oxide ore into a fluidized bed type chloridization furnace maintained at from 800.degree. to 950.degree. C, causing a selective reaction with iron oxides by means of carbon monoxide and chlorine thereby to remove volatile FeCl.sub.3, and continuously taking out the ore in a state wherein less than 8 percent, preferably from 1 to 5 percent, of iron oxides are remaining therein from the chloridization furnace.
As another example, the specification of British Pat. No. 992,317 discloses a similar process wherein a temperature of from 800.degree. to 1,100.degree. C is used in a selective chloridization reaction in a fluidized bed type chloridization furnace. In still another example, the specification of U.S. Pat. No. 2,184,885 discloses the admixing of carbon in a quantity of from 1 to 12 percent by weight relative to the ore and a reaction temperature of from 700.degree. to 1,150.degree. C. Furthermore, the specification of U.S. Pat. No. 2,933,373 specifies, similarly, a carbon quantity of from 20 to 30 percent and a reaction temperature of 1,050.degree. C. On one hand, the specification of Japanese Patent Laid Open No. 2657/1971 discloses a process which comprises taking out ore which has partly undergone a chloridization reaction from a fluidized bed type chloridization furnace, cooling this ore in a reducing atmosphere such as carbon monoxide or methane, and subjecting this ore to magnetic separation thereby to separate it into a TiO.sub.2 portion containing substantially no iron and a portion containing iron.
In each of the above enumerated patents and patent application, an object of the invention is to provide a process for chlorine treatment of titaniferous ores in which the iron oxides within the ore are caused to react selectively with chlorine and thereby to be removed, and the titanium content is left as a residue, the basic principle of the reaction control being the imparting of selectivity to the reaction of the chlorine relative to the iron oxides and the titanium oxide, and, at the same time, to determine the conditions for effective use of the chlorine from the standpoint of economy.
In a reducing atmosphere, however, the iron oxides and titanium oxide both react with the chlorine and respectively become iron chloride and titanium chloride, which are volatile. Accordingly, the reaction for obtaining artificial rutile from a titaniferous ore is based on the utilization of the difference between the reactivities of the iron oxides and of the titanium oxide relative to chlorine. Under ordinary conditions, however, this difference is very slight, and, as the reaction progresses from the ore outer surface toward the interior, the iron oxides in the interior remain in their unreacted state and also the titanium oxide, which is of a residue on the surface, begin to react with the chlorine, whereby the selectivity of the reaction is lost.
As measures conceived heretofore for overcoming this difficulty, there have been the practices of adjusting the quantity of the added coke in accordance with the composition of the starting material ore and the desired TiO.sub.2 grade of the product and of holding the reaction temperature within a range of from 800.degree. to 950.degree. C. However, even when the reaction temperature is held within this range in the chlorine treatment process of the titaniferous ore wherein iron oxides are selectively chloridized, as the reaction is carried forth so as to reduce the residual iron oxide within the ore to less than 6 percent, that is, from 1 to 5 percent as in the specification of Australian Pat. No. 206,305, for example, a large quantity of TiCl.sub.4 is produced. As a result, this gives rise to a loss in the titanium content, a lowering of the chlorine efficiency, and various deleterious effects on the process steps succeeding the chloridization process step.
On the other hand, the reduction of the quantity of the added coke or the carrying out of the reaction at a low temperature, as a measure for repressing the reaction of the chlorine with respect to titanium oxide, lowers the chlorine reaction efficiency and adversely affects the progress of ion-oxide removal. Moreover, a large quantity of chlorine in unreacted state passes through the chloridization furnace.
In contrast, a process wherein, by causing chlorine to react at a high temperature, e.g., 1,050.degree. C, simultaneously in the presence of an ample quantity of a reducing agent, the chlorine reaction efficiency is maintained at a high value, and, moreover, the formation of TiCl.sub.4 can be suppressed is disclosed in the aforementioned Japanese Patent Laid Open No. 2657/1971. According to this process, the ore is taken out of the chloridization furnace in state wherein iron oxides are remaining as residue in a quantity of at least 5 percent, preferably at least 10 percent, for example, 12 percent.
With a residual iron oxide quantity of 5 percent, however, TiCl.sub.4 is formed, whereby a considerable quantity of titanium becomes lost as mentioned hereinabove. Accordingly, the ore is taken out of the chloridization furnace before the reaction progresses to that extent, but the ore thus taken out inevitably contains a large quantity of incompletely reacted ore. If this incompletely reacted ore is separated and repeatedly caused to undergo reaction in the chloridization furnace, a large number of cycles of the chlorine treatment of the incompletely reacted ore of large quantity mentioned above will become necessary before a TiO.sub.2 product of the desired grade can be obtained, whereby the productivity of the chloridization furnace will inevitably drop. Furthermore, the loss of chlorine due to chlorine or chlorides adsorbed by or adhering to the ore taken out of the furnace unavoidably increases with increase in the quantity of ore repeatedly treated in the furnace.